Your current team can offer 8 years vs every other club offering 7. If 5 is the max you get from your current club, then you really have no extra incentive to take the 5 years unless you and your agent agree that is roughly where you would land in free agency.
Like with Klingberg, the offer from DAL was the Max 8 years. So, for him to pass on that offer to get only 7 years as a UFA, was he going to get an extra $700K to $1 mill per season to make up for that 8th year and/or take the risk of hitting the market at age 35 to make up for the remaining difference?
This case with Brock, he didn't get that, so he will have to see what the market bears.